获得字典的补充

Question

I have a dictionary letters

letterstoProbabilityMap={"aaa":0.4,"bbb":0.7,"ccc":01}

for which I have three letter strings and their probability of occurring(I have shortened the dictionary). I am assigning these probabilities based on some training data. But I also want to assign a probability to strings/keys I haven't seen. eg "aaa". Since all my keys are within the set aaa-zzz. Is there a quick way for me to obtain the non assigned/complement and assign a value quickly. (I understand my question is quite abstract.)

EDIT The value is not fixed it is actually a la place probability. Below is a code snippet I use to compute the probabilities I do know The point is I reserve a probability mass which I will then assign to the three letter strings I haven't seen(because I know all strings are between aaa-zzz)

for trigram in sorted(threeletter_counts.keys()):
        numerator=threeletter_counts[trigram]+1 
        denominator=twoletter_counts[trigram[:2]]+30
        prob=numerator/denominator

Answer 1

You could go through all strings and use setdefault :

for letters in itertools.product(string.ascii_lowercase, repeat=3):
    letterstoProbabilityMap.setdefault(''.join(letters),
                                       computeMissingProbability(letters))

Though if the calculation is expensive and would go to waste often because most keys already exist, better check first:

for letters in itertools.product(string.ascii_lowercase, repeat=3):
    key = ''.join(letters)
    if key not in letterstoProbabilityMap:
        letterstoProbabilityMap[key] = computeMissingProbability(letters)

Or maybe use a defaultdict , if that works for you:

fullMap = collections.defaultdict(lambda: 0.123, letterstoProbabilityMap)

If the default value is just 0.0:

fullMap = collections.defaultdict(float, letterstoProbabilityMap)